Optical imaging system

ABSTRACT

An optical system for imaging an input beam from an input to an output beam at an output. The system includes a lens adapted for receiving the input beam from the input, and a reflecting device adapted for reflecting the input beam from the lens back to the lens. The lens is adapted for converging the beam reflected by the reflecting device to the output. The input and the output are both located substantially in the focal plane of the lens and off axis, with respect to the optical axis of the lens, on opposing sides from the optical axis of the lens. The reflecting device is located on a side opposite, with respect to the principal plane of the lens, to the side of the input and the output.

BACKGROUND OF THE INVENTION

The present invention relates to an optical imaging system for opticallymapping an object at an input to an object at an output.

Optical imaging systems are widely known in the art and can be appliedfor mapping an input image to an output image substantially with orwithout modifying the characteristics of the input image. A typicalapplication wherein the characteristics of the input image is intendedto be modified by the optical imaging system is in optical attenuator.

Optical attenuators are known e.g. from DE-A-3613688 or GB-A-2074339orEP-A-557542 discloses an optical attenuator as shown in FIG. 1. Theattenuator is inserted between two optical fibers 1 and 2. A cone oflight 3 leaving the fiber 1 is converted by a lens 4 to a beam ofparallel light 5, which impinges on an attenuator disc 6. The attenuatordisc 6 comprises two parts 25 and 28 glued together such that they forma disc having rectangular cross section. After transmission through thedisc 6, the attenuated light beam 7 impinges on a corner cube 8reflecting the incident light into the same direction where it camefrom, but with a parallel offset. Thus, the beam reflected from thecorner cube 8 is again transmitted through the disc 6, but at a slidedifferent position than at the first transmission.

The purpose of transmitting the beam twice through the attenuator disc 6is to compensate beam deviations caused by refractive index differencesand oblique light incidence. After the second transmission through thedisc 6, the beam 9 impinges on a prism 10 which reflects the beam twiceby 90 degrees such that the outgoing beam 11 has a parallel offsetrelative to the beam 9 leaving the attenuator disc 6. The outgoing beam11 is then focused by a lens 12 into the fiber 2. The disc 6 isrotatable around an axis 13 by means of a motor 14 in response tocontrol signals from a control circuitry 20. The adjustment of differentattenuation factors is achieved by adjusting different angularorientations of the disc 6.

The attenuator disc 6 comprises one part 28 which is made of a lightabsorbing material and one part 25 which is substantially transparentfor the light impinging on it. Those parts are wedge-shaped and fixedtogether such that the resulting disc 6 has a rectangular cross section.The thickness of the wedges, in combination, remains constant in adirection perpendicular to the plane of the paper in FIG. 1. The beams 5and 9 impinging on the light-absorbing wedge from the left or from theright (after reflection by the corner cube 8) traverse the same distancewithin the light-absorbing wedge. Since the thickness of the absorbingpart of the disc 6 which the beam 5 passes depends on the angularorientation of disc 6, the attenuation of the beam 5 can be continuouslyvaried by rotating the disc 6 around the axis 13. The motor 14 might befurnished with a position encoder indicating the angular position of theattenuator disc 6.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved opticalattenuator in particular with respect to costs. The object is solved bythe independent claims. Preferred embodiments are shown by the dependentclaims.

The invention provides an improved optical imaging or mapping system formapping an object at an input source to an object at an output. Theoptical mapping system of the present invention comprises a sourceadapted for emitting a diverging optical beam to a lens, a reflectingdevice (such as a mirror) adapted for reflecting a parallel beam fromthe lens back to the lens, and an output adapted for receiving aconverging beam from the lens. The source and the output are bothlocated substantially in the focal plane of the lens and off axis, withrespect to the optical axis of the lens, on opposing sides from theoptical axis of the lens. The reflecting device is located on oppositeside of the source and the output with respect to the principal plane ofthe lens and arranged so that a beam from the source will be mapped tothe output.

Because of the off axis location of the source and the beam with highestintensity from the source not being directed through the center of thelens, the lens collimates the diverging light beam from the source to aparallel beam skew with respect to the optical axis. The reflectingdevice reflects this skew but parallel beam back to the lens. In thisreverse direction, the optical radiation is focused to the output. Theposition (i.e. distance away from the lens and angle with the principalplane of the lens) of the reflecting device is preferably chosen forachieving a low insertion loss for coupling optical radiation from thesource to the output.

It will be appreciated that while the optical mapping system of FIG. 1comprises two lenses (4 and 12), the optical mapping system of thepresent invention is designed to require only one lens, and the opticalbeam travels twice through the lens system. Thus, the optical mappingsystem of the present invention requires less components and adjustmenteffort than the optical mapping system applied for the attenuator ofFIG. 1, and is therefore less expensive and easier to produce andadjust.

In a preferred embodiment, the plane of the reflecting device issubstantially parallel with the principal plane of the lens. However, itis to be understood that tilting (i.e. providing an angle between theplane of the reflecting device and the principal plane of the lens) thereflecting device can be used/required for compensating a difference inoff axis location between the source and the output, and vice versa.

In another embodiment, the plane of the reflecting device substantiallycrosses the optical axis of the lens at the cross point of the opticalaxis of the lens with beam of highest intensity from the source afterpassing the lens. Thus, a low insertion loss can be achieved.

Another degree of freedom for the design of the mapping system is theangle of radiation (preferably of the beam with highest intensity) fromthe source. Increasing the angle provided with the optical axis of thelens requires that the reflecting device has to be positioned fartheraway from the lens. Thus, the longitudinal dimension of the system canbe adjusted as might be required e.g. for inserting components in thebeam.

It is to be understood that the optically simplest arrangement for amapping according to the invention is in case that:

the source and the output are both located off axis in the focal planeof the lens, whereby the beam with highest intensity from the source isdirected to the lens but not through the center of the lens;

the plane of the reflecting device is substantially parallel with theprincipal plane of the lens and might only be slightly tilted in orderto compensate a (slight) deviation in the distances of the source andthe output from the optical axis of the lens; and

the plane of the reflecting device substantially crosses the opticalaxis of the lens at the cross point of the optical axis of the lens withbeam of highest intensity from the source after passing the lens.

It is clear, however, that deviations from this ‘easiest’ design canincrease the optical complexity of the system, but might be desired forspecific applications.

In a preferred embodiment, fiber ends respectively represent the sourceand the output. The input and output fiber ends, emitting light towardsthe optical attenuator or receiving light therefrom, are preferably bothlocated in the focal plane of the lens. The position of the input fiberend and the output fiber end are both off axis, each preferably situatedwith the same distance away (offset) from the axis. The input and outputfibers are preferably located very tight together and preferably gluedin a holding device. By providing the fiber ends angled, the angle ofradiation of the center beam (with highest intensity) and thus thedistance between the lens and the reflecting device can be adjusted.

Within the optical mapping system of the present invention, differentcomponents (such as any kind of attenuating filter as known in the artsuch as the attenuating filters as disclosed in EP-A-557542) can beapplied for modifying the characteristics of the output beam withrespect to the characteristics of the input beam of the system. Thecomponent(s) is/are preferably inserted between the lens and thereflecting device, whereby the component(s) can be arranged so that thebeam travels once or twice through the component(s), or, in other words,that either only the parallel beam to or from the reflecting device, orboth pass the respective component(s).

The input and output fiber ends can be straight or angled, and might beprovided e.g. by using polishing or cleaving techniques. Angled fiberends can be applied for achieving low back reflections (high returnloss), as disclosed e.g. in the co-pending European Patent Application99109469.9 by the same applicant. The teaching of that document withrespect to the provision of termination surfaces will be incorporatedherein by reference.

Between the lens and the reflecting device, an optional polarizationrotator (as disclosed e.g. by Donald K. Wilson in “Optical isolators cutfeedback in visible and near-IR lasers”, Laser Focus/Electro-Optics,Dec. 1988, pp. 103 ff.) might be located to reduce the polarizationdependent attenuation e.g. resulting from an inserted component such asa filter. Preferably, the polarization rotator is an optical Faradayrotator with 45 degrees polarization rotation. In this case, the opticalradiation forward beam and reverse beam go through the Faraday rotatorand the total polarization rotation is 2×45°=90°. Because of theresulting polarization dependent loss PDL=0, the optical polarizationaxis of any elliptical (or linear) state of polarization of the reversebeam is rotated 90 degrees with respect to the optical polarization axisof any elliptical (or linear) state of polarization of the forward beam.Due to the passing twice through the optical filter with a polarizationrotation of 90 degree between the forward and the reverse beam, thepolarization dependent loss of the optical filter is averaged out. Theresult is a polarization independent attenuation of the complete opticalsystem according to the invention.

The reflecting device is preferably designed to provide a maximum ofreflectivity and a low insertion loss. For wavelengths in the nearinfrared spectrum, e.g. about 1550 nm, the reflecting device ispreferably provided as a gold-coated mirror. Accordingly, for otherwavelengths, a mirror with maximum reflectance will have to be chosen.

The optical system according to the invention can thus be designed toprovide no or only reduced polarization dependent attenuation, inparticular with respect to the embodiment of FIG. 1. Furthermore theinventive optical system is less costly than e.g. the system of FIG. 1,since less material is required (e.g. only one lens instead of two).Additionally, the adjustment effort can be significantly reduced whene.g. the input and output fibers are adjusted together. Moreover, due tothe double-pass design a compact size of the complete optical system canbe achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the attendant advantages of the presentinvention will be readily appreciated and become better understood byreference to the following detailed description when considering inconnection with the accompanied drawings. Features that aresubstantially or functionally equal or similar will be referred to withthe same reference sign(s).

FIG. 1 shows an optical attenuator as known in the art, and

FIGS. 2 and 3 show embodiments of an optical attenuator 100 according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a first embodiment of an optical attenuator 100 accordingto the invention comprising a lens 110, an optical mirror 120 and anoptical attenuating filter 130 in-between. P denotes the principleplane, F the focal plane, and O the optical axis of the lens 110. Theinput fiber 1 and the output fiber 2 are both located off axis,preferably with the same distance (offset) d away from the optical axisO. Furthermore, the ends of the input fiber 1 and the output fiber 2 arepreferably both located substantially in the focal plane F.

In case that the ends of the input fiber 1 and the output fiber 2 arenot off axis with the same distance from the axis, the mirror 120 has tobe tilted by an angle Δα (as indicated in FIG. 2) with respect to theprincipal plane P. The fiber end 2 will be displaced by an amount Δydependent on the angle Δα and the focal length f:

Δy≈f· tan(2·Δα).

The cone of light 3 leaving the fiber 1 is collimated by the lens 110 toa parallel forward beam 135. Due to the off axis location of the inputfiber 1, the forward beam 135 is skew with respect to the optical axisO. The forward beam 135 is reflected at the optical mirror 120, which isprovided substantially parallel to the principle plane P of the lens110, into a reverse beam 140 traveling back towards the lens 110. Theoptical lens 110 focuses the reverse beam 140 back as a converging beam145 into the output fiber 2.

It is clear that only if the ends of the input fiber 1 and the outputfiber 2 are located in the focal plane F, the forward beam 135 will befully parallel. Although a certain non-parallelism of the forward beam135 will be acceptable for most applications, the deviation from thefocal plane F should preferably be smaller than approximately 5%(preferably about 2%) of the focal length f. Otherwise, the opticalarrangement will become increasingly complicated.

Since the optical attenuating filter 130 is situated between the lens110 and the optical mirror 120, as well the forward beam 135 as thereverse beam 140 will pass therethrough, so that both beams 135 and 140can be attenuated. The attenuating filter 130 can comprise anyattenuating means as known in the art such as the attenuating disc 6 asdescribed in detail in the above cited EP-A-557542. The opticalattenuating filter 130 can be provided rotatable, preferably around theoptical axis O, or fixed. Instead of changing the degree of attenuationby rotating an attenuating filter as disclosed in EP-A-557542, alongitudinal attenuating filter can be applied, whereby the degree ofattenuating is varied by moving the optical attenuating filter 130parallel to the principle plane P of the lens 110. Attenuating filterswith attenuation coating on a substrate (e.g. metallic coating) can beapplied accordingly.

Between the optical attenuating filter 130 and the optical mirror 120,an optional polarization rotator 150 might be provided for reducing thepolarization dependent attenuation of the optical attenuator 100.Preferably, the polarization rotator 150 is an optical Faraday rotator(as disclosed e.g. in EP-A-352002) with 45° polarization rotation, sothat the total polarization rotation is 2×45°=90°.

FIG. 3 shows a second embodiment of an optical attenuator 100 accordingto the invention, which substantially corresponds to the embodiment ofFIG. 2. The input fiber 1 and the output fiber 2, however, are providedwith angled fiber ends 200 and 210. The angle of the fiber endsdetermines the angle of radiation of the center beam (as normally thebeam with highest intensity—indicated in FIGS. 2 and 3 by a dottedcenter line in the light paths) and thus the distance between the lens110 and the mirror 120. Increasing the angle of radiation will alsoincrease the distance between the lens 110 and the mirror 120, thusallowing to achieve more space for inserting components between the lens110 and the mirror 120. It is clear that the angle of the fiber end 210has to be opposite to the angle of the fiber end 200, as depicted inFIG. 3.

In a preferred embodiment, the input fiber 1 and the output fiber 2 aredirectly coupled together, whereby a surface 230 at which the inputfiber 1 and the output fiber 2 are mated together is locatedsubstantially within the optical axis O.

The fiber ends 200 and 210 are preferably angled towards each other ascan be seen from FIG. 3. Preferably, the fiber ends 200 and 210 are eachangled with respect to the optical axis O in a range smaller thanapproximately 10°, and preferably about 8°.

What is claimed is:
 1. An optical system for imaging an input beam froman input to an output beam at an output, comprising: a lens adapted forreceiving the input beam from the input, and a reflecting device adaptedfor reflecting the input beam from the lens back to the lens, whereby:the lens is adapted for converging the beam reflected by the reflectingdevice to the output, the input and the output are both locatedsubstantially in a focal plane of the lens and off axis, with respect toan optical axis of the lens, on opposing sides from the optical axis ofthe lens, the reflecting device is located on a side opposite, withrespect to a principal plane of the lens, to the side of the input andthe output, and wherein an angle of radiation from the input providedwith respect to the optical axis of the lens is adjusted for adjustingthe distance between the reflecting device and the lens.
 2. The opticalimaging system of claim 1, wherein the reflecting device is asubstantially plane mirror for reflecting a substantially parallel beamfrom the lens back to the lens.
 3. The optical imaging system of claim1, wherein a reflecting plane of the reflecting device is substantiallyparallel with the principal plane of the lens.
 4. The optical imagingsystem according to claim 1, wherein a plane of the reflecting devicesubstantially crosses the optical axis of the lens at a cross point ofthe optical axis of the lens with a beam of highest intensity from theinput after passing the lens.
 5. The optical imaging system according toclaim 1, wherein: the input and the output are both located off axis inthe focal plane of the lens, whereby the beam with highest intensityfrom the input is directed to the lens but not through the center of thelens; the plane of the reflecting device is substantially parallel withthe principal plane of the lens or slightly tilted in order tocompensate a deviation in the distances of the input and the output fromthe optical axis of the lens; and the plane of the reflecting devicesubstantially crosses the optical axis of the lens at the cross point ofthe optical axis of the lens with beam of highest intensity from theinput after passing the lens.
 6. The optical imaging system according toclaim 1, wherein: the input includes a first fiber end and the outputincludes a second fiber end, wherein the first and second fiber ends arelocated together in a holding device.
 7. The optical imaging systemaccording to claim 6, wherein the fiber ends are angled for adjusting adistance between the lens and the reflecting device by means of theangle of radiation.
 8. The optical imaging system according to claim 1,wherein: a polarization rotator is arranged between the lens and thereflecting device, so that an optical beam from the input to the outputtravels at least once through the polarization rotator, for reducingpolarization dependent attenuation.
 9. An optical modification systemfor modifying the characteristics of an optical beam, comprising: anoptical imaging system for imaging an input beam from an input to anoutput beam at an output, including: a lens adapted for receiving theinput beam from the input, and a reflecting device adapted forreflecting the input beam from the lens back to the lens, whereby thelens is adapted for converging the beam reflected by the reflectingdevice to the output, the input and the output are both locatedsubstantially in a focal plane of the lens and off axis, with respect toan optical axis of the lens, on opposing sides from the optical axis ofthe lens, the reflecting device is located on a side opposite, withrespect to a principal plane of the lens, to the side of the input andthe output, and wherein an angle of radiation from the input providedwith respect to the optical axis of the lens is adjusted for adjustingthe distance between the reflecting device and the lens; wherein theoptical beam of the optical modification system is applicable at theinput thereof, the optical modification system further comprising: afilter arranged between the lens and the reflecting device of theoptical imaging system, so that the optical beam travels at least oncethrough the filter.
 10. The optical imaging system of claim 1, whereinan angle is provided between the plane of the reflecting device and theprincipal plane of the lens for compensating a difference in off axislocation between the input and the output.
 11. The optical imagingsystem according to claim 8, wherein the polarization rotator is anoptical Faraday rotator with 45 degrees polarization rotation.
 12. Theoptical modification system of claim 9, wherein the filter is anattenuator.